A high-capacity, sophisticated, and economic access service system is demanded in association with recent rapid diffusion of the Internet, and research of passive optical network (PON) is underway as means for realizing it. The PON is a system of inserting a branching unit in the middle of an optical fiber network to bring one optical fiber into a plurality of subscriber homes and an optical communications system to share one center device and a part of a transmission path by a plurality of users by using an optical multiplexer/demultiplexer by an optical passive element for an economic purpose.
In Japan, an economic optical communications system to share a 1 Gbps circuit capacity by up to 32 users by time division multiplexing (TDM) and a so-called Gigabit Ethernet™ PON (GE-PON) are currently mainly introduced. According to this, a FTTH (fiber to the home) service is provided at a realistic cost.
In order to respond to larger capacity needs, research of 10GE-PON with a 10 Gbps-level entire bandwidth is underway as a next-generation optical access system and international standardization thereof was completed in 2009 (21st year of Heisei). This is the optical communications system which realizes the large capacity while using the same transmission path such as the optical fiber as that of the GE-PON by increasing a bit rate of a transmitter-receiver. It is considered that a 10 G-level or larger capacity will be demanded for an ultra high-definition video service, a ubiquitous service and the like in the future, but there is a problem that realization thereof is difficult due to increase in cost for system upgrading only by simply increasing the bit rate of the transmitter/receiver from a 10 G level to a 40/100 G level.
A wavelength-variable WDM/TDM-PON in which wavelength variability is added to the transmitter/receiver and the above-described time division multiplexing (TDM) and wavelength division multiplexing (WDM) are effectively combined such that the transmitter/receivers in a center station device may be expanded gradually according to bandwidth requirement is reported (for example, refer to Non-Patent Literature 1) as technology to solve such problem. In realization of a system of such wavelength-variable WDM/TDM-PON, an unused wavelength is used such that this may coexist with a conventional system, and for realizing the same at a low cost, it is considered to use a wavelength-variable optical transmitter of a broad wavelength interval of 1.3 μm band in a user side device and use a wavelength-variable optical transmitter of a narrow wavelength interval of 1.5 μm band on a center device side. A wavelength optical multiplexer/demultiplexer is required for performing wavelength distribution of the signal light with the broad wavelength interval of 1.3 μm band and the signal light with the narrow wavelength interval of 1.5 μm band, so that it is considered to use the arrayed waveguide grating as such wavelength optical multiplexer/demultiplexer.
The arrayed wavelength grating being the wavelength optical multiplexer/demultiplexer is formed of an input waveguide, an input side slab waveguide, an output waveguide, an output side slab waveguide, and a channel waveguide group connecting the input side slab waveguide and the output side slab waveguide, and the channel waveguide group is formed of a plurality of channel waveguides having path lengths sequentially becoming longer by a predetermined path length difference (refer to Patent Literatures 1, 2, or 3, for example). Such arrayed waveguide grating may set the wavelength band and wavelength interval according to the system to which this is applied and the arrayed waveguide grating of the 1.3 μm band broad wavelength interval, the arrayed wavelength grating of the 1.5 μm band narrow wavelength interval and the like are reported, for example. However, the wavelength interval of the arrayed waveguide grating depends on the path length difference of the channel waveguide group, so that the arrayed waveguide grating capable of distributing the signal light with the broad wavelength interval in the 1.3 μm band and that with the narrow wavelength interval in the 1.5 μm band cannot be realized by the conventional technology.
Therefore, it is considered to use the arrayed waveguide grating having a function to perform the wavelength distribution of the signal light of the 1.3 μm band (wavelength band A) with the broad wavelength interval (wavelength interval X) and the arrayed waveguide grating which performs the wavelength distribution of the signal light of the 1.5 μm band (wavelength band B) with the narrow wavelength interval (wavelength interval Y) in combination as the arrayed waveguide grating capable of distributing the signal light with the broad wavelength interval in the 1.3 μm band and that with the narrow wavelength interval in the 1.5 μm band and connect input/output ports of the arrayed waveguide gratings through optical filters, and this is examined.
FIG. 22 is a schematic diagram of an example of a conventional wavelength distributor H in which two arrayed waveguide gratings 302 and 303 and a number of optical filters 301 and 304 connecting the input/output ports are combined. Such wavelength distributor H is formed of a plurality of optical filters 301, the arrayed waveguide grating 302, the arrayed waveguide grating 303, a plurality of optical filters 304, and optical fibers 307 for connecting the optical filters 301 and 304 and input waveguides or output waveguides of the arrayed waveguide gratings 302 and 303.
Meanwhile, in the following description, the signal light incident from a side of an input/output port 305 of the wavelength distributor H to be emitted to a side of the input/output port 306 is referred to as an upstream signal and the signal light incident from the side of the input/output port 306 to be emitted to the side of the input/output port 305 is referred to as a downstream signal. As the signal light, a case in which the number of the input/output ports 305 and the number of the input/output ports 306 are N, the signal light of the wavelength band A {λa1, λa2, . . . λaN; wavelength interval X} is on a shorter wavelength side than the signal light of the wavelength band B {λb1, λb2, . . . λbN; wavelength interval Y}, and the wavelength interval X is larger than the wavelength interval Y is described as an example, and a case in which the optical filters 301 and 304 are formed of thin-membrane filters (optical interference filters) having characteristic illustrated in FIGS. 2 and 3 described later (for example, refer to Patent Literature 4) is described.
The signal light {λb1, λb2, . . . λbn} incident from I1 of the input/output port 305 passes through the thin-membrane filter in an optical filter 301-1 and arrives at the arrayed waveguide grating 302 for the narrow wavelength interval. Each signal light is distributed to optical filters 304-1 to 304-n by the arrayed waveguide grating 302. Each distributed signal light passes through the thin-membrane filter in the optical filter 304 to be emitted to O1 to On of the input/output port 306.
The signal light {λb1, λb2, . . . λbn} incident from I2 of the input/output port 305 passes through the thin-membrane filter in the optical filter 301-1 and arrives at the arrayed waveguide grating 302 for the narrow wavelength interval. Each signal light is distributed to the optical filters 304-2, 304-3, . . . , 304-n, and 304-1 by a cyclic property of the arrayed waveguide grating 302. Each distributed signal light passes through the thin-membrane filter in the optical filter 304 to be emitted to O2, O3, . . . , On, and O1 of the input/output port 306. The same applies to the signal light {λb1, λb2, . . . λbn} incident from I3 to In of the input/output port 305 and this is wavelength-distributed as illustrated in FIG. 23(b) described later.
The signal light {λa1, λa2, . . . λan} incident from I1 of the input/output port 305 is reflected by the thin membrane filter in the optical filter 301-1 and arrives at the arrayed waveguide grating 303 for the broad wavelength interval. Each signal light is distributed to the optical filters 304-1 to 304-n by the arrayed waveguide grating 303. Each distributed signal light is reflected by the thin membrane filter in the optical filter 304 to be emitted to O1 to On of the input/output port 306.
The signal light {λa1, λa2, . . . λan} incident from I2 of the input/output port 305 is reflected by the thin-membrane filter in the optical filter 301-1 and arrives at the arrayed waveguide grating 303 for the broad wavelength interval. Each signal light is distributed to the optical filters 304-2, 304-3, . . . , 304-n, and 304-1 by the cyclic property of the arrayed waveguide grating 303. Each distributed signal light is reflected by the thin-membrane filter in the optical filter 304 to be emitted to O2, O3, . . . , On, and O1 of the input/output port 306. The same applies to the signal light {λa1, λa2, . . . λan} incident from I3 to In of the input/output port 305 and this is wavelength-distributed as illustrated in FIG. 23(a) described later.
Meanwhile, when the signal light is incident from the side of the input/output port 306 to be emitted from the side of the input/output port 305 by reversibility of a travel direction of light, the above-described wavelength distribution may also satisfy a correspondence relationship in FIGS. 23(a) and 23(b) to be described later.
In this manner, the upstream signal of a short wavelength band with the broad wavelength interval and the downstream signal of a long wavelength band with the narrow wavelength interval are wavelength-distributed bi-directionally over a signal fiber by the wavelength distributor H which is illustrated in FIG. 22 as an example.